1. Field of the Invention
This invention relates to method and apparatus for removing cyanide based contaminants and recovering heavy metal such as copper, silver, nickel, iron, cobalt and the like from waste water streams, and more particularly to a process for removing cyanide base contaminants and heavy metal components from an effluent stream by adsorption of the heavy metal on a medium and thereafter stripping the adsorbed metal from the medium for recovering by electrolytic deposition.
2. Description of the Prior Art
The treatment of heavy metal and cyanide bearing effluent streams to prevent the discharge of pollutants in waterways has become the subject of federal legislation. The Environmental Protection Agency (EPA) has been charged with enforcing effluent limitations in publishing guidelines for the discharge of electroplating wastes into waterways. The need for such legislation has resulted from the use of copper, silver and the like in metal treating operations, such as plating, anodizing, conversion coatings, corrosion inhibition, pickling, and ore processing. These limitations also extend to the discharge of other plating solutions containing cyanide, copper, silver, nickel, iron, cobalt, and other multi-valent metals.
It is known to remove cyanide bearing compounds and heavy metal contaminants from waste water streams by performing a pH adjustment on the waste water stream in the range of 9-10. An oxidizing agent such as hypochlorite and sodium hydroxide is fed into the waste water stream to oxidize the cyanide bearing compounds. This reaction oxidizes the cyanides to cyanates (CNO) with complete oxidation obtained when the cyanates are further oxidized to carbon dioxide (CO.sub.2) and nitrogen gas (N.sub.2). Since this reaction is pH dependent, oxygen is also required in the reaction. Chemical precipitation is performed on the oxidized mixture to remove the heavy metals resulting in a heavy metal sludge material which requires disposal. Also, the remaining effluent is not suitable for recycling.
Another process for removing cyanide bearing compounds from waste water includes catalytic oxidation as disclosed in U.S. Pat. No. 5,635,078. The waste water is passed through a bed of an adsorbent catalyst, such as activated carbon, and a small amount of heavy metal is added to the stream. In the presence of cyanide and oxygen, the metal present in the stream reacts to give up electrons to drive the oxidation process. The metals are then adsorbed onto the activated carbon bed. The cyanide is oxidized by the reaction to nondetection levels, effectively removing the cyanide from the waste water.
While various methods are known to treat cyanide and heavy metal bearing effluents for the oxidation of the cyanide and the removal of metal contaminants, the known devices not only require substantial capital investment for equipment costs but create a secondary problem in the disposal of the recovered pollutant, particularly for disposal of the sludge which is extracted from the recovery process. The sludge must be handled in a manner that meets the requirements of disposal of solid wastes. Metallic sludges when mixed with garbage and organic wastes are likely to go back into solution as organic acids. When the pollutant is recovered as a metallic salt strict procedures must be followed to process them as a sanitary waste. Therefore there is need for method and apparatus in the treatment of cyanide and heavy metal bearing effluents that eliminates the problems associated with toxic metal sludges and produces an effluent stream which is environmentally safe.
There is a further need for a cyanide catalytic oxidation process which permits recovery of heavy metals present in the effluent stream, extends the operative life of the bed of adsorption material and produces an effluent which is suitable for recycling to the process area.